One promising means of recovering oil from oil shale is to retort the shale in situ. Currently, modified in situ technology requires the construction of extensive underground openings or retorts. The remaining media (structure left around these retorts must support the overburden and contain the retorting shale. A failure criterion for oil shale, which is presented in this paper, was developed so that regions in the retort structure where the shale may be approaching failure can be identified. This criterion, adapted from composite materials applications, is essentially a closed surface in six-dimensional stress space and makes it possible to describe the anisotropic nature of failure in layered materials. The failure surface can be defined by five parameters which are determined from five simple laboratory tests. The surface is developed for a 80 ml/kg kerogen content shale and its features are discussed in detail. The predictions of the model are found to be in agreement with the results of a large number of laboratory tests, including uniaxial and triaxial compression tests. One unique (for rocks) test series is discussed which involves the failure of thin-walled tubes under combined compression and torsion. Finally, it is shown how the model can be extended to include the variation of material properties with kerogen content and temperature.